A conventional differential device known as a differential gear is so composed that the respective wheel shafts of left and right drive wheels are differentially rotated by the arrangement of gears forming a planetary gear so as to equalize the arithmetic mean of the sum of angular velocity of the wheel shaft of the drive wheel at one side and that of the wheel shaft of the drive wheel at the other side or the difference therebetween.
Such a conventional differential device performs the differential operation in angular velocities of the respective wheel shafts of the left and right drive wheels, since a difference between the resistances applied to the left and right wheels is caused by the deviation of a steering wheel due to its steering operation. Therefore, such a conventional differential device is manufactured to be adapted to a wheel type vehicle having a steering wheel, while it is difficult to be adapted to a crawler type vehicle having no steering wheel. Further, the differential operation of the angular velocities of the left and right wheel shafts is performed by the difference between the travel resistance applied to the left drive wheel and that applied to the right drive wheel. Thus, if one of the left and right drive wheels falls into a muddy place free from resistance, for instance, only the drive wheel losing the resistance rotatably slips in its spot, and the other drive wheel having the resistance does not rotate to thereby make the travel of a vehicle impossible.
The crawler type vehicle having no steering wheel is controllably steered by driving the respective wheel shafts of the left and right crawlers by individual hydraulic motors, and controlling the operation of the hydraulic motors by the operation of a control lever to change the rotational speed of the left and right crawlers. For this reason, the vehicle of crawler type is inferior to the vehicle of wheel type in smoothness of the steering operation and the controllability.